When using a thermal printhead, the problems associated with mounting the printhead to ensure even printing across the full row of printing elements are well known. As used herein, the phrase "even printing" means that each printing element will produce a substantially equally dark dot when fully energized. The aforesaid known problems are especially troublesome in the case of wide printheads, i.e., on the order of 10 inches. Such printheads are desirable, however, as they can print all the characters of an entire line of an ordinary 81/2.times.11 or A4 sheet of paper simultaneously. Uneven force of the printhead against the print-receiving medium can create varying contrast with light or dark zones within the printed line.
When the thickness of a print-receiving medium, such as heat-sensitive paper, is uneven, the printhead mounting arrangement also must be able to compensate for the variations in paper thickness while maintaining even printing.
U.S. Pat. No. 4,855,756 discloses an apparatus comprising a combination of two springs, sliding frames and a computer-controlled cam. The printhead in this apparatus is mounted on a holder that is pivoted on a shaft and supported by a C-shaped spring. The C-shaped spring biases the holder to a position parallel to a print-receiving medium. When an uneven medium is placed under the printhead, the C-shaped spring deforms irregularly. This may lead to uneven pressure on the medium. Thus, this apparatus, while perhaps compensating for some variations in thickness, is complex and would require extensive disassembly for maintenance and repair.
U.S. Pat. No. 4,235,555 discloses a printer having multiple printheads wherein each printhead is balanced by applying pressure at one central point. The head is otherwise connected at two nearby points by a resilient bracket, with deformation of the bracket allowing for minor variations in the paper. Again, however, the system involves several small printheads, movable brackets, arms, cams, springs and levers, which make it very complex to produce and difficult to repair and maintain.
Thus, it is an object of the invention to provide an improved printhead mount that causes the force between the printhead and the print-receiving medium to be substantially equal along the width of the printhead.
It is another object of the invention to provide a printhead mount that can compensate for variations in the thickness of a print-receiving medium, even where such variations occur across the width of the medium.
It is another object of the invention to provide a printhead mount that meets the foregoing objects, yet is easily engaged and released, allowing for simple maintenance and repair.
It is still another object of the invention to provide a printhead mount that is economical, both in the number and cost of the component parts, and is simple to manufacture.